Radiotransmission



rJuly 3, 192s.

R. A. HEISING RADIO TRANSMISSION 5 Sheets-Sheet l Filed March 19, 1924 mg l July 3, 192s. 1,675,888

R. A. HElslNG RADIO TRANSMISSION Filed March :1.9, 19244 s sheets-sheet 2 Julys, 1928. 1,615,888

R. A. HEISING RADIO TRANSMI SSION Filed March 19, 1924 5 Sheets-Sheet 3 Patented July 3, 1928.

UNITI-:o Ns'mres 1,675,888 PATENT ori-fics."

RAYMOND HEISING, OF MILLBURN, NEW JERSEY, ASSIGNOR T0 WESTERN ELECTRIC COMPANY, INCORPORATED, 0F NEWYORK, N. A CORPORATION 0F NEW YORK.

RADIOTRANSMISSION.

Application led March 19, 1924. Serial No. 700,386.

This invention relates to high power radio systems and has for one object to transmit speech efficiently over long distances.

A further object is to provide improved methods of transmission, especially radio telephone transmission.

This object is attained by modulating in accordance with the speech to be transmit-ted a low power electrical wave of a frequency several times that of speech, selecting by a filter the lower side band from the resultant wave and ut'ilizing'it to modulate a second low power electrical wave which has a frequency a few times that of the speech modulated frequency. The lower side band is selected by a filter from the resultant wave obtained by the second modulation and is transmitted to an antenna through a series of amplifiers ranging from an output of a few watts up to several kilowatts.

The'frequency of the first carrier wave is so selected that the lower side band is of a frequencyv sufliciently low to be easily separated from the modulating frequency 'Y and theother side band. The second frequency of wave' to be modulated is so selected that the lower side band of the resultant waveV is sufficiently removed from the carrier frequency and upper side band that there is no difficulty in selecting the desired band and there is a `considerable range between the upper side band of the first modulat-ion and the second carrier. frequency.

This doublemodulation process provides considerable flexibility in the frequency range and permits the use of a second filter of a wide range. Furthermore, the frequency arrangement is maintained in the transmitted band in the same order in which it occurs in the voice, This prevents trouble from harmonics in case the balance of the system is not perfect. v

Advantage is also taken of the fact that while by far the larger part of the energy of speech is concentratedl in frequencies below 500 cycles, these frequencies have little eect upon the articulation and that for a range of frequencies from 500 to 3200- cycles the energy content is approximately only 25% of the total speech energy, and yet provides over 75% of the articulation with the conversational intelligibility about 98% perfect. By utilizing this restricted range only a trifle over one-tifth of the power is :necessary that would be required if the lfull speech range 'were transmitted and this makes possible a greater number of channels in Vthe same frequency range.

Referring now to the drawings, Figs. 1, 2 and 3 disclose successive portions of a svstem embodying the invention. n

In the drawings, a source of speech waves 10, represents, for example, a telephone line, connected to a subscribers transmitter 7. A microphone circuit- 8 is provided for the operator of the system to talk if he desires. A two-way switch 9 is provided for opening and closing the circuits 10 and 8. In its 1 n1ddle position, it opens both circuits. In its right-hand position, it closes the subscribers circuit and leaves the operators v.circuit open, while in its left-hand position` 1t opens the subscribers circuit and closes the operators circuit. Monitoring can be accompllshed in the usualway by connecting an operators receiver and subset across the line incoming from the subscribers transmitter 7. Circuit 10 is inductively connected through the transformer 11 with a balanced modulator circuit 12. An oscillator circuit 13 is provided for supplying to the modulator 12 oscillations of low carrier frequency, for example, of the order of 33,700 cycles. The oscillator 13 comprises an electron discharge device 14, the filament of which has its opposite ends connected to the ends of a secondary of a transformer 15, the primary of which is supplied with alternating cur-- rent from an ordinary lighting circuit of cycles through a variable resistance 15a. The midpoint of the secondary of the transformer 15 is connected through a conductor 16 and condensers 47 of one microfarad each to each end of the cathode. Space current for the tube 14 is supplied from two direct current 110 volt generators g and g in series, the positive brush of generator g bemg connected to the anode of the tube 14 through conductor 16a, resistance 17 of 125 ohms, choke coil 18 having a value of .28 henries at 33,000 cycles, the negative brush of generator g being connected to the midpoint of the transformer 15 through conductor 19 and conductor 16. A by-pass to conductor 1G is provided through resistance 20 of 200 ohms shunted by condenser 20u of one microfarad. The oscillating circuit may be traced from the anode through resistance 21 of 48,000 ohms, condenser 22 of ,01 microfarads, the oscillating circuit comprising oscillator coil Y23 of 15 millihenries, shunted by an adjustable condenser 24 having a maximum value of .0015 microfarads, a iixedair condenser 24a of .0007 microfarads and'conductor 16 to the mid-point of the scc-V ondary of transformer 15. The grid circuit of the oscillator may be traced from the grid of the tube through condenser 25 of .00015 microfarads, shunted by resistance 26 of 480,000 ohms, induct-ance 27 of .approximately the same value as coil 23 and conducf conductor 30, primary of transformer 31 and conductor 19. The gridofeach vacuum tube 32 ofthe modulator. 12 is connected in series with a ysecondary yvindingof transformer 31 and onehalf the secondary of transformer 11 through condenser. 33. 1 One secondary Winding of transformer 31 is shunted by-a condenser 31a of .00025 microfarads and the other is shunted by a variable condenser 31b having a maximum value of .00075 microfarads. f The cathodes of the tube 32 are connected in multiple with. each: other and to the opposite ends of the secondary of transformer 34, the primary of which is supplied with alternating current from an ordinary lighting system of 60 cycles through the rheostat The mid-point of the secondary of the transformer 34 is connected to ground throughthe conductor 19 and to the ends of the filament througlrcondensers 36 of 1 microfarad each. Space current for the anode circuits of the modulator issupplied from the generators g and g, through conductor 16a resistance 37 of 350 ohms, potentiometer 33 of 2,000 ohms,and one Winding of the primaryof differentialv transformer 39, a by-pass to conductor 19 being provided through condenser 40 of l1 microfarad, shunted by a resistance 41 of 630 ohms'.- The transformer 39 is` inefficient for the transmission of speech frequencies. A 20 vvolt polarizing battery 42 has itsv positive terminal connected to the conductor 19 and its `negative terminal to conductor 43 connected tothe mid-point of the secondary of *transformer 11, whereby the negative po tential .is impressed upon the grid of each tube 32.

rEhe secondary of the differential transy former l39 is connected to a band filter 44,

which is designed to transmit freely frequencies from 30,500 to 33,200, and to transmit With some attenuation frequencies of 500 to600 cycles higher. The output side of the filter 44 isconnected to the primary of a transformer 45,the secondary of Which forms a part of the input circuitvof a second balanced modulator.

A second oscillator 46 is provided for gen erating oscillations of a higher frequency than that generated by oscillator' 13, for erf ample, of the order of 89,200 cycles. The cathode of the tube 46 has its terminals connected directly to the ends of the secondary of transformer 15 and to the conductor 16 through condensers 47 of 1 microfarad each.

Space current for the anode circuit of the tube 46 is supplied from the generators g and gl through conductor 16, resistance 4S of 125 ohms, cholre coil 49 of appro.\:in1at \ly 100 millihemues` at 89,000 cycles, a by-pass to conductor 16 being provided by a resistance 50 of 700 ohms, shunted hy a condenser 50 of 1 microfarad. The oscillating circuit may be traced from the anode of tube 46 through `the resistance 51 o'l 43,000 olnns, condenser 52 of .01 micra'larad, oscillator coil 53 ol'i 2 millihcnries, shunted by a variable capacity 54 having a miximum of .0015 microilarad, and conductor 16. The grid circuit of the oscillator may be traced from the grid through a condenser 55 of .00015 microfarad, shunted by a resistance 56 of 480,000 ohms, inductance 57 of approximately the same value as oscillator Coil 5e and conductor 16.

Oscillations generated by the oscillator 46 are impressed on the grids of tubes 63 by means of a potentiometer arrangement consisting of resistances 59 and 61 through the secondary of transformer 45. The circuit may be traced through condenser 5S of .0l mocrofarad, resistance 59 of 36,000 ohms, conductor 60 tapped on to the mid-point olf the secondary of the transformer 45, resistAn ance 61 of 48,000 ohms, and conductor 19, a

condenser 62 of one microfarad being bridged across the battery 42.

. The grids of thetubes 63 of the secondL balanced modulator 64 are polarized negatively by the battery 42. The ends of the cathodes of the tubes 63 are connected directly to the opposite ends of the secondary of the transformer 34 and through condensers 65of 1microfarad each to conductor 19. Space current is supplied for the output cil cuit of the modulator from generators ,f/ and g through conductor 16a, resistance 65 of 350 ohms, potentiometer 66 of 400 ohms, and one primary Winding of the differential transformer 67, a path being provided from the potentiometer to conductor 19 through condenser 68 of 1 microfarad, shunted by the resistance 69 of 700 ohms.

The secondary of the transformer 67 is connected to the input of a second band lilter 70 so designed as to have a transmission band of from 41,000 to 71,000 cycles. rl`hc output of filter 70 is connected to the primary of a transformer 71. The grid of a ES Watt amplifier 72 is connected in series with the secondary of the transforn'ier 71 through tap 73a, a portion of the resistance 73, conductor 74 to the positive pole of the lll() i eo A microfarad.

generator g', the other end of resistance 73 being connected to the conductor 19 and ground. The tap 73 is so placed that 62 ohms are included in the grid circuit. The cathode ofamplifier 72 has its ends connected through resistances 75 of 24 ohms each to the ends of the secondary of a transformer 76, the primary of which is connected to an ordinary lighting circuit of 60 cycles through a rheostat 7 6, and also has its ends connected through condensers 77 of one microfarad each to the mid-point of secondary of transformer 76 by the conductor 74. As the mid-point of the cathode is connected through a portion of resistance 73 to the grid, the latter is negatively polarized. A condenser 78 of one microfarad is bridged from the tap 73 to one side of the secondary of transformer 76. Space current for the anode circuit of the amplifier 72 is supplied from a 1500 volt generator 79 through switch 80, conductor 80, resistance 81of 10,000 ohms and choke coil 82 of .02 henries,l a by-pass is provided by condenser 83 of .015 microfarads shunted by a resistance 84 of 10,000 ohms.

The anode of the amplifier 72 is connected to the gridsof a 50 watt amplifier 85 through the condenser 86 of .015 microfarad capacity. The cathode of amplifier has its ends connected to the opposite sides of the secondary of the transformer 76 through resistances 87 of .56 ohms and through the condensers 88 of one microfarad capacity each and the conductor 74 to the mid-point of the secondary of the transformer 76. The grid circuit of amplifier 85 comprises a resistance 89 of 12,000 ohms and the 107 ohm portion of resistance 73 between tap 7 3b and conductor 74, which is shunted by a condenser 90 of one microfarad. As the conductor 74 is connected to the resistance 73, the grid is negatively polarized. Space current for the anode circuit of amplifier 85 is supplied from generator 79 through resistance 92 of 10,000 ohms and speech frequency choke coil 91 of .02 henries, a. by-pass circuit being provided to conductor 74 through condenser 93 shunted by variable resistance 93a.

Amplifier 94 comprises three 250 wett vacuum tubes in parallel, the grids of which arel connected to the anode of amplifier 85 through condenser 95 of .015 microfarad. The grid circuit comprises speech frequency choke coil 96 of .2 henries, resistance 97 of 2,000 ohms and a 210 ohm portion of resistance 73 between tap 73c and conductor 74, which is shunted by condenser 98 of one The filaments of the tubes of amplifier 93 are connected in parallel and have their respective terminals connected directly to the ends of the secondary of transformer 76 and through the condensers 99 of one microfarad to the mid-poiiit of the secondary of said transformer through the conductor 74. As the conductor 74 is connected to the resistance 73, the grid of the tube is negatively polarized. Space current for the anode circuit of the tubes of amplifier 94 is supplied from generator 79 through conductor 80, speech frequency choke coil 100 of .04 henries, the inductances 101, aby-pass being provided to conductor 74 through condenser 102 of .015 microfarad. The output of the series of amplifiers is impressed upon a potentiometer 103 through the condensers 104 of one microfarad each, the potentiometer being grounded at 105.

The operation of the system up to this point may be described as follows: Low power oscillations of carrier frequency are generated by the oscillator` 14 and are impressed upon the two grids of the balanced modulator 32 in phase and are modulated by audio frequency waves impressed upon the modulator'from the telephone circuit 10, the amplitude of the carrier and speech waves being materially the same. Preferably, the oscillations generated by the oscillator 14 are of 33,700 cycles. This figure is not arbitrary, but it is highly desirable that the oscillations be of this order and the system will be described on the assumption that the oscillator is operating at the frequency mentioned.

There is thus produced in the output circuit of the modulator, oscillations of 33,700 cycles, 33,700+S, 33,700-S and S, the speech frequency being denoted by S. As speech frequencies do not ordinarily run much higher than 3,200 cycles, the limit of the band of carrier frequencies' in the output circuit will be from 30,500 to 36,900. Because the differential winding of the transformer 39 and its inefficiency for transmitting speech frequencies, the 33,700 component will be eliminated from the circuit and the speech coinponent greatly reduced, while theV upper and lower side bands will be transmitted to the filter 44. As frequencies below 500 have practically no effect upon articulation and the major portion of the energy of speech is below 500 cycles, it is quite practical and desirable largely to eliminate these frequencies from the band to be transmitted. The filter is therefore designed to have a theoretical upper cut-oif at 33,200, but does not, however. cut off sharply at this point. The attenuation begins to increase rapidly at this point so that frequencies to af few hundred cycles higher, while not entirely eliminated, are reduced considerably in amplitude, attenuation, being practically complete at 33,800. The lower cut-off is around 30,500. This filter, therefore, passes all the lower portion of the lower side band and passes the upper portion of the side band well enough to retain a sufficient amount ofthe frequencies resulting from the low voice frequencies to retain part of the speakers voice characteristics. From the filter 44 the lower side band passes to the cycles.

'71,000 although it may be narrower.

second modulator 64, which is also of the balanced type, in order to reduce the amplitude of the second carrier and prevent overloading the filter 7 0. The oscillator 46 supplies a second carrier at a frequency of 89,200 The modulating frequencies which. now range between 30,500 and 33,200 are impressed in opp site phase on the .grids of the two modulating tubes. The 89,200 cycle carrier is impressed on the two grids in phase. As the transformer 6T is differentially connected inthe output circuit of the modulator, it transmits the side bands and the modulating frequencies buteliminates the carrier. The two side bands pass from the modulator into-the second filter 70, the transmission band of which is from .1-1,000 to Y Only the lower side band falls within this range, the first modulating frequency beingbelow it and the second modulatingfrequency and upper side band being well above it. This filter is so designed that its impedance is very low -in the neighborhood of 30,000 to 33,000 cycles. The resistance outside the band ig practically zero, and the reactance curve crosses the axis at this point. A filter having a reactance characteristic of this 4 type allows the modulators to function most efficiently since in order to get the most ,modulated power out of the modulator shown, the impedance of the plate circuit for both the modulating and modulated. frcquencies'must be low. It is notnecessary to Inakethe impedance of the filter, either zero or a`mininmm .for the modulated frequency of` 89,200, as the differential connection of lthe transformer in effect eliminates the filter from-.the circuit. The saine requirements hold in the case of the first modulator. The differential transformer connection eliminates the filter from the circuit for the carrier yfrequency but not for the speech frequencies.

If thev filter has not the desired low `impedance Vat speech frequencies,y `z idvantfure may be taken of the inefficiency of the transformer at these :frequencies to provide .the

,frequency would be kept out of the amplifier and the antenna. The degree to which this Inustbe kept out is very great for the reason that the -set is of high power and it does not take a very large .input to put several watts into an antenna even though it is off tune. The highest position where the de- The frequency of y sired side band may possibly be placed is around 08,000 to 71,000 cycles. The frequency from the second. oscillator would then be about l0l,000 cycles. The upper side band in both of these caseswould be at 100,- 000 cycles or above. The second filter will easily eliminate these upper side bands.

The second filter is built to have a very high attenuation between 24,000 and 35,000 cycles because it is in this region that the modulatingy frequencies of the fond modulator lie. The arrangement of the second modulator is such that the modulating fretuiencies pass through both tubesl and the second transformer readily into the second filter so that it is of importancethat th(` second filter should eliminate them for they can produce in succeeding amplifier tubes se'fond harmonies which might lie directlyv in the range of the final side band. 'lhat is, the first band lies between 30,500 and 33,200, the second harmonics from it would, lie between 01,000 and 00,400, and if it were attempted to use this latter region as the `position of a side band for communication,

it might be found that some of the harmonics would fallV within the band and give extraneous disturbing noises. These noises would actually be more ,disconcerting than strays and other noises because they would occur simultaneously with words coming in and have a proportionately varying amplitude. i

As it is necessary to replace the carrier at the receiving station, it is essential that the frequency of the carrier waves at the sending station remainconstant. As an oscillator ordinarily changesy its frequency, when either the plate voltage or filament voltage varies, means are taken to prevent either voltage variation andso to minimize frequency variation. The principal cause of trouble is the variation in the impedance of the tube, either plate to filament .impedance or grid to filament resistance. In order to minimize the plate to filament impedance, a very high resistance is placed between the plate and the tuned circuit so that the variation. in impedance of the tube will be a small part of the double impedance to which the timed circuit is attached. The use of the resistance in this position necessarily means that the efficiency of the oscillator will be low. To counteract this and. yet get sufficient voltage for operating the device, the tuned circuit is built with as low a resistance as possible, thereby allowing the building up of sufficient voltage for operating the modulator. Y

lThe grid to filament space resistance also gives trouble due to its variation chiefly as a function of the filament current variation. 'ihe `grid to filament space resistance enters into the operation of the vacuum tube in the form of an absorber of energy. This energy llt) loss must, therefore, be reduced to a minimum. This is accomplished by rising a stopping condenser and grid leak resistance of such a magnitude that the grid current is very small. The variation in losses with variation in grid to filament inipedauce corresponding small and the e'liect on vthe frequency is reduced. Grid resistances of the order of 100,000 ohms are used. These oscillator circuits maintain their frequency constant within a few hundredths of one per cent over practicallya hundred per cent variation in plate voltage or the variation in filament current running from the regular operating point to the slightly oscillating point. In order to keep the temperature variation down, air core inductances and choke coils are used entirely, as iron core coils give too great a variation.

Frein the second modulator the desired side band is passed 'into the three-stage low power amplifier, consisting ofthe amplifiers 72, 85 and 94. The actual power secured in the various amplifier stages is probably lower than that of the rating of the tubes. As power efficiency in this part of the set is not of importance but quality is, these three stages are built for reproducing the desired band faithfully even at a sacrifice of power. The last twofstages are purely voltage step-up amplifiers or choke coil amplifiers. The power secured'from the last amplifier is probably not much more than 500 watts.

The' system of double modulation produces a side band for transmission of high quality and one in which thefrequency arrangement is in the same order in which it occurs in the voice. Because of the relatively narrow band selected, approximately only 1/5 of the power is necessary for transmission that would be necessary if the full speech range were transmitted.

The band to be transmitted is impressed upon the grids of two vacuum tubes forming a power amplifier through conductor 106. Filament current is supplied to the tubes of the amplifier from an ordinary 60 cycle lighting circuit through the transformer 111, the circuit being grounded at 112. ,Polarizing potential for the grids is supplied through conductor 108 from the direct current generator 107, thepositive terminal of which is grounded. Space current is supplied for the tubes from an alternating current generator 113 through a bank of rectifiers 114, choke coil 115, resistance 116 and inductance 117. The output circuit of the amplifier is connected by a step-down transformer 118 to the input circuit of a second power amplifier 119 which may consist of as many power tubes as is necessary to obtain the desired power, three only being shown in this instance. Filament current is provided for the tubes of this amplifier system from i tem 119 feeds into a tuned circuit 132 which isinductively connected at 133 to the antenna circuit 134. The antenna used in practice is of the multiple tuned type.

A circuit breaker 135 is provided for opening the alternating current circuit in the event of any trouble with the power amplifying system. The circuit breaker 135 is held in circuit closing position by the solenoid 136 which energized from the generator 137. A circuit 138 is provided for opening the holding circuit in the event of overloading of the generator 113. A circuit is provided for permitting the holding circuit to open at 139 in the event that thereY is any decrease in the flow of cooling fluid for the anodes of the tubes of the power amplifying system, this circuit being controlled by the switch 140. A third circuit is provided for opening `the holding circuit at 141 or 142 in the event of the overloading of the rectifier-s or the anode or the DC .supply for the anode circuits of the tubes. A lamp 143 indicates the position of the circuit breaker 135. The circuit through the lamp Y143 is maintained closed through engagement of arm 144 with contact 145 when the circuit breaker is in circuit closing position. An arm 146 actuates a contact 147 to open or close circuits through the lamps 148, 149 or other signaling means.

A test oscillator circuit 150 for developing oscillations of 1,500 cycles 1s provided for testing the output of the power amplifier circuit and may be included in the power amplifier circuit by operation of the switch A and 151. p

The receiving circuit for use in conjunction with the transmitting circuit herein described is disclosed on page 124 of the April, 1923, number of the Proceedings of the Institute of Radio Engineers. This circuit comprises an antenna for impressing` the received waves upon a beating oscillator having a frequency of 89,200. The oscillator feeds into a high frequency detector, the output circuit of which feeds through an intermediate frequency filter to an amplifier circuit in which the frequencies transmitted by the filter are amplified. From the amplifier the band is demodulated by a `wave having a frequency of 33.700 cycles supplied by local oscillator circuit. A low vfrequency detector is provid edr for detecting the resultant wave of the modulation. With the system described in this application there is substantially no disturbance caused in the vreceiving set duc to ripples introduced in the transmitted band from the use ofan alternating current genL erator as a source of supply for the plate current of the poweramplifier system. Because of the fact that the space current for the amplifier is supplied through aplural phase rectifier, the ripple impressed upon the transmitted band from the plate filament circuit will be .several :times the frequency of the filament supply sources. Thus, instead of transmitting asingle. band, there will be transmitted ya band having upper-and lower side hands. The successive modulations in the receiving circuit will therefore produce speech tones together with upper and lower sidel bandsor ripples. Since with the single side,r band ,transmitting system there is no output from the transmitting station except when speech -is impressed, .the ripples will occur in the receivingset only at the same time that vspeech is being received. The noisetherefore, from the ripples will Abe inappreciable as it is .very small in ycomparison with the speech `and .occurs in conjunction with speech.

Reference should vleehad to thecopending applications of Oswaldv Serial No. 562,973, filed May 23, .1922,ffor a more detailed description .of the -high power amplifier and Oswald Patent 1,535,180, `issued April 28, 1925, for a :complete disclosure of the operation of thev high power amplifier and antenna. l

What` is claimed is: l

1. A modulating system comprising a source of speech waves, means for modulating said speech waves with a carrier wave of relatively low frequency, means for selecting and transmitting one side band of the lresultant wave, .means `for modulating said side .band with a carrier wave of higher frequency thany said first carrier wave, and means for selecting. one side band from the second modulated wave comprising a :[ilter having la transmission range intermediate the limit of :theupper side band ofthe first modulated wave andthe second `carrier frequeney.l

f2.k A modulating system comprising a source of speech waves, :means for modulating with saidk speech waves, a carrier wave of higher frequency thanY speech, means for selecting one kside band from the resultant wave, means formodula-ting said selectedvba-nd with a second wave of more than twice the frequency of said second wave and means for selecting one side :band from the second modulated wave comprising a filter having high attenuation at frequencies corresponding to those of the .first selected side band and of the second carrier wave, and being adapted to transmit freely substantially all waves of frequencies therebetween.

3. A modulating system .comprising a source of speech waves, a source of carrier frequency waves, means for modulating with said carrier wave, said speech wave, means for selecting and transmitting one side band of the modulated wave, a secmid source of carrier waves, means for varying the frequency of the waves generated therein,

means for modulating with the waves from said second source, said first selected side band, and means for selecting one side band of the second modulated wave comprising a band-pass,` lter the transmission range of which is substantially equal to the range from Zero to the first carrier frequency.

4. A modulating system comprising a source of speech waves, means for modulat ing a relatively low carrier wave with said speech waves, a filter having a relatively narrow transmission range for selecting a band from the resultant wave, meansl for modulating a second carrier wave with a band transmitted by said filter, and a second filter having a transmission range several times that of said first filter.

5. The method of transmitting speech by electric Waves which vcomprises transmitting,r with substantially no attenuation waves corresponding to tones higher than 50() cycles per second, and partially suppressing the waves corresponding to tones lower than 500 cycles, the suppression increasing as the tone becomes lower.

6. The method of transmitting speech by electric waves which comprises modulating a carrier wave in accordance with speech, suppressing the carrier wave and one side band and partially suppressing the waves of the other sideband corresponding to the lower speech tones.

7. In a plural modulation wave transmission system a plurality of carrier wave Sources comprising space discharge oscillators having frequency determining circuits operatively associated therewith for the generation of continuous waves, a common source vof space current for said oscillators, and means for stabilizing the frequency of the generated waves comprising a conductive shunt path in parallel with the space discharge path Aof each oscillator, and a resistance in series in the current supply lead to each oscillator from said space current source.

In witness whereof, I hereunto subscribe my name this 19th day of March, A. D., 19,24.

RAYMOND A. HEISING. 

